Nodejs how to access callback variable outside the function? - node.js

Question

This is my code want to access callback variable newID outside calling-function. I want to insert bulk data into mongodb using batch with auto incremented id instead of default object
for (var i = 0; i < sizeOfResult; ++i) {
var newKey = {}; //Main json array
newKey = {
date: result[i]['date'],
issue: result[i]['issue'],
status: result[i]['status']
};
getNextSequenceValue("inventoryid",db, function(err, newID) {
newKey["_id"] = newID; <!-- try to add/assign callback variable(newID) into newKey -->
});
console.log("newKey: %j", newKey); <!-- but unable to get access callback variable(newID) here below-->
batch.insert(newKey);
}
// This is my called function
function getNextSequenceValue(name,db,callback) {
var ret = db.collection('counters_inv').findAndModify({ _id: name },null,{ $inc: { sequence_value: 1 } }, {new: true},
function(err,doc ) {
if(err){
return callback(err) // callback on error
}
callback(null, doc.value.sequence_value); // callback on success
});
}

Answer 1

Look at this code, you just need to put the variable outside and it works:
let i = 1
function functionwithcallback(callback) {
console.log(i)
i++
callback(i)
}
for (let j = 1; j <= 10; j++) {
functionwithcallback(() => {
if (j == 10)
console.log('done')
})
}

Answer 2

I'm not sure what the overall objective is, but the reason your newKey variable is not set correctly is because where it is being used has executed before the variable is set. In your example, your for loop is going to completely finish running kicking off a bunch of getNextSequenceValue() method calls that will eventually come back and run the callback code. It does not wait on the getNextSequenceValue function to finish before continuing the loop.
Solution: moving the console.log() and batch.insert() into the callback.
Here's an example that would execute in the correct order.
var keys = [];
for (var i = 0; i < sizeOfResult; ++i) {
var newKey = {
date: result[i]['date'],
issue: result[i]['issue'],
status: result[i]['status']
};
getNextSequenceValue("inventoryid", db, function(err, newID) {
newKey["_id"] = newID;
keys.push(newKey);
if (keys.length === sizeOfResult) {
console.log("keys: %j", keys);
batch.insertAll(keys);
}
});
}
function getNextSequenceValue(name, db, callback) {
db.collection('counters_inv').findAndModify({ _id: name }, null, { $inc: { sequence_value: 1 } }, {new: true},
function(err,doc) {
if(err){
return callback(err);
}
callback(null, doc.value.sequence_value);
});
}